The present invention relates to a press that is set up for cutting thick and/or, more particularly high-strength sheet metal as well as a method for operating such a press.
When punching or cutting high-strength sheet metal, great fluctuations in forces occur between the ram and the punching die over time and can also change suddenly in particular. As long as the material of the workpiece resists the stamp, a very high force prevails, so that some parts of the press undergo elastic deformation. This pertains to the press bed, the punching die, the press stands, the press head piece and to a certain extent also the ram plus the connecting rod and the eccentric shaft. If the workpiece yields under the influence of the stamp, then the energy stored elastically in the aforementioned elements is released in a relatively uncontrolled manner.
To be able to monitor this process better, DE 102 52 625 A1 proposes a system for reducing the cutting shock, in which a number of hydraulic cylinders are provided in the die. These may be arranged above, below or at the side of the workpiece. Sensors such as ultrasonic sensors or sensors that measure the flow rate of hydraulic fluid flowing out of the hydraulic cylinders cause the closing of a valve through which the hydraulic fluid could previously flow out of the hydraulic cylinders. The hydraulic cylinders are connected to pressure accumulators that are under a relatively high pressure. They therefore now generate a high counterforce. The force previously exerted by the stamp on the workpiece is thus transmitted at that moment to the hydraulic cylinders in which the stamps begin to penetrate through the workpiece.
This method of dampening the cutting shock has proven fundamentally successful, but the adjustment of the sensors for detecting penetration of the workpiece is critical. Even when the hydraulic cylinders are arranged next to the workpiece, there is still a certain cutting shock which should be further reduced.
Against this background, an object of the present invention is to improve upon the state of the art as defined above.
This object has been achieved with a press having a controllable supporting device that is set up for generating a variable force that is effective between the ram and the press bed, and a control unit that is assigned to the supporting device and detects by means of a sensor device a quantity that is in a unique correlation with the ram position and that influences the force exerted by the supporting device and acting between the ram and the workpiece as a function of the quantity and a method whereby force applied by the supporting device us varied as a function of a quantity related specifically to ram position.
The press according to the present invention has a supporting device that is active between the ram and the press bed and creates a force acting between them. For example, the supporting device is part of the sheet metal holding device that presses the die against the lower die during the forming operation. For example, the lower die is a punching die, and the upper die is a stamp. The sheet metal holding device, which is configured as a supporting device, is capable of exerting various forces. A control unit assigned to the supporting device is capable of influencing the force exerted by the supporting device.
The control unit influences the force exerted by the supporting device according to this invention on the basis of a parameter that is uniquely related to the position of the ram. This parameter may be, for example, the ram position itself or, as is preferred, the press angle or some other measured quantity. If the press angle is used as the basis, then a press driven by a rotating shaft, such as that with eccentric presses, toggle presses or the like, is assumed. The rotational position of the drive shaft, in particular the eccentric shaft, is referred as the “press angle.” The press angle (or the other parameter uniquely characterizing the position of the ram) with which a sudden increase in the counterforce exerted by the supporting device is preferably to take place is, for example, predetermined and preset by the die manufacturer. It is contemplated to provide for this preset value of the press angle at which the reversal of force takes place to be corrected in braking in the press, i.e., to be varied on a trial basis to achieve the smoothest possible operation of the press in the sense of maximum dampening of the cutting shock. The value thus set is preferably saved and then used for subsequent operation of the press.
The set value for the press angle may be defined specifically for a given die and workpiece. Such values may be stored in a retrievable form in a table, so that new settings for the press angle at which the reversal of force takes place with the supporting device will be available in retrofitting the press.
Due to the reversal of the supporting force that is performed in the simplest case exclusively on the basis of the press angle, the press may easily be set for different particulars, especially with regard to the number of strokes or the cutting force.
As a rule, a hold-down plate, which is supported directly on the workpiece, belongs to the sheet metal holding device. The hold-down plate extends to the immediate vicinity of the stamps (punching stamps) and thus approaches the cut to be produced in close proximity. The sheet metal should thus be clamped tightly in the immediate vicinity of the cut between the hold-down plate and the lower die (punching die) to achieve a high-quality cut. With the press of the present invention, the sheet metal holding device or some other supporting device preferably receives the force applied by the ram after the stamp breaks through the workpiece while the ram is passing through its bottom dead center and saves the energy thereby emitted by the ram in between. In the return stroke of the ram, this energy is delivered back to the ram and thus to the press drive. By avoiding in this manner, uncontrolled release of the energy stored elastically in the press, the press drive is relieved on the whole, i.e., energy is saved.
The mechanical load on the press is reduced by avoiding excessive sudden changes in force with the present invention. In addition, due to the transfer of force exerted on the ram to the sheet metal holding device until the penetration of the workpiece, an especially tight clamping of the workpiece is achieved precisely during penetration, so that especially high cutting qualities are achieved. Furthermore, the force may be introduced over an especially large area by way of the sheet metal holding device and may thus be introduced gently into the workpiece, so that unwanted deformation thereof, e.g., pinching and the like, can be avoided.
It is also contemplated to detect and monitor the characteristic of the ram force as a function of time or as a function of the press angle. If it goes beyond a predetermined tolerance range as a function of the press angle, the reversal point in time (reversing press angle) for the counterforce applied by the supporting device may be shifted forward or in reverse. This allows a high number of strokes in particular to be achieved.
The sheet metal holding device and/or the supporting device preferably has a hydraulic cylinder that is connected to a first and a second hydraulic pressure accumulator(s). Both of the pressure accumulators have, for example, a displaceably mounted piston with a dampened end stop. Diaphragm accumulators or accumulators in which a gas pressure cushion is directly connected to the hydraulic fluid may be provided as an alternative. Both pressure accumulators preferably have different resting pressures. The path leading from the hydraulic cylinder to the pressure accumulator with a low pressure is preferably regulated by an electrically controlled valve that is controlled by the control unit.
The reversal of the supporting force, preferably from a lower value to a higher value, at a predetermined press angle allows starting of the reversal operation of the hydraulic valve shortly before the press angle at which the counterforce applied by the workpiece collapses, e.g., because the punch punctures through the material of the workpiece. The press angle difference by which the hydraulic valve is opened in the leading phase can be referred to as the “correction angle.” With this angle, time delays caused by the reversal of the hydraulic valve and by the delayed response of other components can be compensated effectively. In contrast with that, systems that use parameters characterizing the collapse of force on the workpiece for reversal of the hydraulic valve can only respond subsequently, i.e., with a time lag. The present invention thus permits effective compensation of the cutting shock that would otherwise occur, especially with a rapid operating speed of the press (high number of strokes).